In a known design, two parallel rows of four cells each are positioned on either side of a chimney with a rectangular section extending over the whole length of the rows. The large length of the size of the chimney imposes a thick chimney duct in order to withstand the significant flexure produced by the pressure of the gases, and therefore the significant weight.
A design derived from the previous one is shown in EP 0 473 498 A. The single chimney is divided into four chimneys with circular section, each of which faces a cell of each of both rows. In other words, each small chimney resulting from the division is placed between two cells. The chimneys are of a circular shape and therefore better withstand the internal pressure of the gases. However, as this solution remains a heavy solution, it is proposed that chimneys be built in composite material in order to reduce their weight, but it is doubtful that a significant gain in weight may be achieved since these chimneys can no longer be thermally assembled to the remainder of the structure and should therefore themselves withstand the forces to which they are subject.